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The Quaternary geology map of Flanders

(Mostaert F., Jacobs P., De Ceukelaire M.)

Rationale

The Quaternary deposits in Flanders are characterised by lateral and vertical heterogeneity and by large thickness variations because they have been deposited at a large extend under terrestrial conditions (e.g. river and dune deposits). In fact most sediment packages have a thickness between 1 m and 30 m at a maximum. At date no maps exist that illustrate the complete Quaternary cover of the Flemish subsoil on an appropriate scale because the old geological map of Belgium (completed by 1919 at a scale of 1/40.000) paid little attention to the Quaternary.

Mapping principles

The systematic Quaternary mapping commissioned by the Ministry of the Flemish Community started in 1993, and is based on the following principles:

  • all available and known cored wells;
  • geotechnical data like cone penetration tests;
  • geophysical data like geo-electrical soundings, geophysical well logs, seismic data;
  • studies of outcrops;
  • interpretations of scientific work like theses and publications;
  • models for extra- and interpolation of data;
  • expertise and experience of Quaternary field geologists;
  • geomorphology and landscape indications.

The mapping itself is based on the technique of profile types. The various zones on the Quaternary geology map represent areas where similar successions of specific Quaternary deposits (profile types) occur. The thickness of the various constituting strata of a profile type is indicated through the additional contour and isopach maps and through the explanatory text. The basic units considered are characterised by their lithology, sedimentary structures, fossils, the interpreted sedimentary environment and a chronostratigraphical indication. Although the mapping is mainly based on lithostratigraphy, there is a certain chronostratigraphical connotation as the mappable units are situated in a time framework.

 

Also a (quite incomplete) set of geomorphological maps is available, initially at a scale of 1/25.000, later at a scale of 1/50.000. These maps accentuate the relief forms, and provide an indication on the origin of the landscape and on the age of its relief forms. As most of the existing relief in Flanders dates from the Quaternary, geomorphological maps provide important information on its Quaternary geological setting.

Applications

The Quaternary geology map has a large added value:

  • it generates additional information for administrations, political authorities and decision makers in the format of policy-supporting scientific documents in various domains like environment, survey and planning, construction and infrastructure;
  • it creates new opportunities for education in geology, geography, survey and planning, agriculture, geotechnics and mining, and even in geography teaching in secondary schools;
  • it provides a solid scientifically based information to the citizens, the consultants, the environment responsible, etc.

At date, mostly the soil map and the groundwater vulnerability map are consulted for an environment assessment report. The Quaternary geology map provides additional information on thepresence of natural resources, on groundwater vulnerability, on future slope stability, on agricultural value and groundwater household, etc.


The digitised profile-type system allows generating custom-tailored specific thematic maps like:

  • geotechnical thematic maps for compaction or stability prognosis;
  • exploitation maps of surficial natural resources like sand, loam, clay, peat or water;
  • environment assessment maps for waste disposal, agricultural exploitation and land use, water extraction, survey and planning.

Lithostratigraphy

A lithostratigraphic description of all Quarternary deposits, together with their age, stratotype, thickness and location can be downloaded here. ( pdf 119 kB - new window)

 

The Tertiary geology map of Flanders

(Jacobs P., De Ceukelaire M., Sevens E., Mostaert F.)

Rationale

The last geological map of Belgium (with a topographical background from 1895) was completed in 1919 and thus largely outdated. The new edition differs from the previous one in many aspects:

  • the new map is a true Tertiary geology map indicating the Tertiary formations outcropping at the Pre-Quaternary erosion surface (i.e. the Quaternary deposits are not indicated on the new map unlike the old one as these Quaternary formations are represented on a new Quaternary geology map);
  • the topographical background is the most currently available one on a 1/50.000 scale;
  • the legend of the new Tertiary geology map is of a lithostratigraphical type (litho units), rendering the new map more user friendly and practical.

Mapping principles

The new Tertiary map at a scale of 1/50.000 is published by the Ministry of the Flemish Community in co-operation with the Belgian Geological Survey (BGS). The Geology Departments of the Universities of Ghent and Leuven have assured the compilation of the geological data.

 

Each printed geological map is accompanied by a set of thematical maps in transparent overlays (like location maps of points of observation with indication of their nature, contour maps of the Pre-Quaternary erosion surface, isopach maps of the Quaternary deposits, contour maps of important formation boundaries, etc.). Three to four sections illustrate the geological architecture of the Tertiary, while all relevant details (like stratigraphy, legend, hydrogeology, maps and sections, excursion route, deeper Tertiary, Mesozoic and Palaeozoic information, geophysical and geotechnical characteristics, natural resources, …) are discussed in an apprehensive explanatory textbook (in Dutch).

 

The French, English, German and Spanish translations of the legend are printed on the backside of each map.

Organisation

An important step in the drawing of a geological map is the collection and the storage of all information available. All data from the BGS archives were completed with the archives of the Geological Departments of the Universities of Ghent and Leuven,consisting mainly in lithological and stratigraphical descriptions of wells and geophysical well logs. Also data provided by drillings and geotechnical (mostly cone penetration) tests from the archives of the Geotechnics Division of the Ministry of the Flemish Community (formerly called State Institute for Geotechnics) have been added to the database. Further use has been made of university (licentiate and Ph.D.) theses, publications, public domain reports of geological surveys, etc.

To store all data orderly and efficiently, preference has been given to a computerised relational database. A PC-application originally in MS-DOS offered a lot of possibilities, became common use and was very user friendly.

 

The database has been organised in modules of Dbase4 tables because of their relatively easy use. Software programmes enable consultation of information in the tables, borehole description reconstruction, etc.

 

The database is composed of the following tables:

  • a table with standard information on each observation point: identification number, topographical information, observation characteristics (nature, drilling technique, depth,...) and reliability identifications;
  • three tables with data on the smallest litho unit identified for each observation point (individualising a deposit!)
    • depth level of top and thickness of the litho unit;
    • lithological description;
    • stratigraphical interpretation.
  • a table with the geophysical parameters grain size, resistivity, geotechnical properties,...).

 

All available information relevant to the observation point is stored into the database. Special attention is paid to the information reliability (-lithological descriptions of cored wells are much more reliable than descriptions of flushed wells-). Data with a low reliability index should be handled with care if used for mapping.

 

After database storage, lithology and location of every observation point are compared with the original files. The stratigraphical interpretation is matched to the most recent and current criteria and adapted if necessary. All this data (corrections included) are added to the database.

Mapping procedure

After the stratigraphical data interpretation, the thickness of the Quaternary deposits in the different observation points is known as is the Quaternary-Tertiary boundary level at which the Tertiary sediments are recorded for the first time. Contour lines interconnect points with the same elevation on the top Tertiary topography depicting its relief.

 

For every observation point the levels of the upper and lower boundary surface of every lithostratigraphical unit are determined. The upper boundary surface of the unit first observed under he Quaternary cover might be absent and the lower surface of the last observed unit might be missing. The upper boundary surface of a unit coincides with the lower one of the overlying (younger) unit. All levels of the lower surface of each differentiated litho unit are represented on a map with an adequate scale. Isohypses interconnect points where the same litho unit is present at the same elevation. The isohypse (or contour) map thus describes the nature of the lower boundary surface of the unit.

 

The contour maps of the top Tertiary topography and of the lower boundary surface of the different lithostratigraphical units form the basal documents for the new geological map. The interconnection line of all intersection points between the boundary surface contour lines with the corresponding contour lines of the top Tertiary topography defines the geological boundaries. During this procedure all information concerning all observation points is constantly evaluated, its reliability checked and its stratigraphical interpretation adapted if necessary.

 

For further reading and details about the maps, the reader is referred to *.

Validation of the maps and the data

The new maps and the associated data are validated by a steering group of experts. Finally the data are checked, converted and integrated in the ‘Databank Ondergrond Vlaanderen’ (or Database of the Subsoil of Flanders). The map and explanatory text are officially published. The digital information, including the maps, is permanently updated. Regular revisions of the maps are foreseen.

 

Lithostratigraphy

A lithostratigraphic description of all Tertiairy formations, together with their age, stratotype, thickness and location can be downloaded here. ( pdf 298 kB - new window)

 

*Jacobs P, De Ceukelaire M, Sevens E, Verschuren M (1993) Philosophy and methodology of the new geological map of the Tertiary formations, Northwest Flanders, Belgium. Bull Soc belge Géol 102 (1-2):231-241.

 

 

 

Geological subcrop map of the Brabant Massif

Introduction

This is a subcrop map of the Lower Palaeozoic rocks, except for two small areas near Halle and Mark where these rocks crop out. The cover of these rocks consists of Quarternary, Tertiairy, Cretaceous and Devonian deposits. This map is a presentation of the hard core of Flanders that is built up of Cambrian, Ordovician and Silurian deposits.

The currently available geological subcrop maps of the Brabant Massif are those of Legrand (1968) and De Vos et al. (1993). Other authors published structural maps of the Brabant Massif (e.g. Mansy et al., 1999; Sintubin & Everaerts, 2002). Many of these maps testify of different views on the architecture and deformation history of the Brabant Massif. This lack of consensus and the important amount of new data, accumulated since 1993, formed the incentives to remap the Brabant Massif on The Flemish Territory.

Mapping principles

  • Geological Lower Paleozoic subcrop map of the Brabant Massif on Flemish Territory.
  • Scale 1/200 000.
  • The mapping project started in 2003 and has reached completion in 2005.
  • The Belgian Geological Survey (GSB) created the map, assisted by a steering committee consisting of members of the Catholic University of Leuven, Ghent University, VITO, ANRE and DOV.
  • Project was funded by ANRE.
  • Aim: a conceptual map, with only of well-founded structures.
  • The map is a Lower Palaeozoic subcrop map, except for two small outcrop areas near Halle and Mark. The cover consists of Cretaceous, Tertiary and/or Quaternary deposits. The parts of the Brabant Massif covered by Devonian deposits are poorly explored and are therefore excluded from the area that will be mapped.

Objectives

At the start of the project, the current concepts on the Brabant Massif were evaluated. One of the main objectives was to create a geological conceptual map, based on the raw data and on well-founded structures only. The goal was to create a map that reflects the present rocks on a certain subcrop-level, as much as possible apart from any large-scale (evolving) concepts.

Mapping Procedure

The mapping project was divided into four steps, each supported by the steering committee.

  • in a first step, all existing data were verified. This involved inventorying all existing descriptions, and checking these where possible with samples in the collections of the GSB. All lithostratigraphic interpretations were re-interpreted according to the latest lithostratigraphic scale (Verniers et al., 2001). A total of ~800 observation points were identified, of which ~500 could be interpreted on formation level;
  • in a second step, the data were visualised as a point map. This is a map showing the available data and data density, without any interpretation;
  • in a third step, a distribution map was drawn based on the point map. Such a map shows the likely interpolation between the observation points. Interpretation is limited to assuming an overall WNW-ESE trend of the distribution pattern. Contacts between formations are not interpreted, although possible anomalous contacts (absence of formations) may be identified on such a map;
  • in a fourth step, existing hypotheses, mainly the structural maps of different authors, were tested by overlying them on the distribution map. This, combined with a critical evaluation, showed that most structures proposed in literature were poorly supported by direct geological evidence;
  • the fifth step involves drawing a conceptual geological map that is primarily based on hard-rock data. Secondary sources of information, such as aeromagnetic data and gravimetric data, are integrated to complete the map. This is preferably done by first interpreting aeromagnetic features using the conceptual map, and only then deducing geological structures from the aeromagnetic map. When (part of) the conceptual map is finished the implications and probabilities of the hypotheses underlying the preliminary map are evaluated and compared. If the concept is rejected, step five is reiterated.

This process results in a new geological subcrop map of the Brabant Massif that will be the reflection of an objective evaluation of all data available and existing interpretations.

Validation of the map and the data

The steering group consisted of experts of the University of Leuven and Gent, employees of VITO, BGD and ANRE. Their duty was to validate the maps and the data and to make sure that the objectives of the mapping project were respected during the process.

All available data of the Paleozoicum of different sources (GSB, DOV, SCKCEN,…) were collected in a database, verified and reinterpreted. Those data, together with the gravimetric and aeromagnetic maps of Flanders, form the basis on witch the map has been created.

All data relevant for DOV (map and interpretations) are checked, converted and integrated in DOV. These data will be updated in the future with new relevant interpretations.

 

Lithostratigraphy

A lithostratigraphic description of all Lower-Paleozoic formations, together with their age, stratotype, thickness and location can be downloaded here. ( pdf 237 kB - new window)

References:

  • De Vos W., Verniers J., Herbosch A. & Vanguestaine M. 1993. A new geological map of the Brabant Massif, Belgium. Geological Magazine, 130: 605-611.
  • Debacker, T.N., Herbosch, A., Verniers, J. & Sintubin, M. 2004 (in press). Faults in the Asquempont area, southern Brabant Massif, Belgium. Netherlands Journal of Geosciences/Geologie en Mijnbouw, 83.
  • Legrand R. 1968. Le Massif du Brabant. Mémoires pour servir à l'explication des Cartes géologiques et minières de la Belgique, 9, 148 pp.
  • Mansy J.L., Everaerts M., & De Vos W. 1999. Structural analysis of the adjacent Acadian and Variscan fold belts in Belgium and northern France from geophysical and geological evidence. In: Palaeozoic to recent tectonics in the NW European Variscan Front Zone. (Ed.: Sintubin, M., Vandycke, S. & Camelbeeck, T.). Tectonophysics, 309, 99-116.
  • Sintubin M. & Everaerts M. 2002. A compressional wedge model for the Lower Palaeozoic Anglo-Brabant Belt (Belgium) based on potential field data. In: Palaeozoic Amalgamation of Central Europe, Geological Society, London, Special Publications, 201: 327-343.
  • Verniers J., Herbosch A., Vanguestaine M., Geukens F., Delcambre B., Pingot J.-L., Belanger I., Hennebert M., Debacker T., Sintubin M. & De Vos W. 2001. Cambrian-Ordovician-Silurian lithostratigraphical units (Belgium). In: Guide to a revised stratigraphical scale of Belgium. (Ed.: Bultynck, P. & Dejonghe, L.). Geologica Belgica 4.